This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
| n | a(n) |
|---|---|
| 1 | 15 |
| 2 | 63 |
| 3 | 511 |
| 4 | 1023 |
| 5 | 16383 |
| 6 | 32767 |
| 7 | 2097151 |
| 8 | 4194303 |
| 9 | 33554431 |
| 10 | 67108863 |
| 11 | 8589934591 |
| 12 | 17179869183 |
| 13 | 34359738367 |
| 14 | 274877906943 |
| 15 | 549755813887 |
| 16 | 70368744177663 |
| 17 | 562949953421311 |
| 18 | 2251799813685247 |
| 19 | 36028797018963967 |